Image data correcting device for correcting image data to remove back projection without eliminating halftone image

ABSTRACT

An image data correcting device eliminates a back projection image without eliminating an image corresponding to a low-intensity halftone image. An intensity difference detection part detects an intensity difference between first image data corresponding to a part of a predetermined small area and second image data corresponding to the remaining parts of the predetermined small area. A halftone detection part determining whether the first image data corresponds to a halftone image. An intensity change part changes an intensity of the first image data to a predetermined low intensity, when the intensity difference is equal to or smaller than a first predetermined value and the first image data does not correspond to the halftone image and the intensity of the first image data is equal to or smaller than a second predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to image forming apparatusesand, more particularly, to an image data correcting apparatus applicableto an image reading device or an image forming apparatus which canremove data corresponding to an image printed on a reverse side of aprinted paper.

2. Description of the Related Art

When reading a both-side printed material such as newspaper, timetable,magazine, etc. by using a scanner or a copy machine, an image printed ona back side of the printed material may be undesirably read when readingan image on the front side of the printed material. Such a phenomenon isreferred to as “back projection” or “reverse-side projection”.

Japanese Laid-Open Patent Application No. 3-068270 discloses an imageprocessing apparatus that detects low intensity image data andcalculates an average value to be set as threshold value so that imagedata having an intensity value below the threshold value is changed toimage data corresponding to white.

Japanese Laid-Open Patent Application No. 8-340447 discloses an imageforming apparatus which comprises back projection area determiningmeans, back projection level detecting means, back projection levelcorrecting means and back projection level correction value storingmeans so as to eliminate an image caused by a back projection.

Japanese Laid-Open Patent Application No. 9-116753 discloses an imagereading apparatus which eliminates an image corresponding to a backingby previously reading a surface of the backing and subtracting imagedata corresponding to the backing from image data obtained by reading adocument.

Japanese Laid-Open Patent Application No. 11-055524 discloses an imageforming apparatus which calculates MTF of image data, and obtains anaverage value of the MTF so as to determine that the image datacorresponds to a back projection when the average value is below asetting value, and applies a background process to the image data.

The methods disclosed in Japanese Laid-Open Patent Application No.3-068270 and Japanese Laid-Open Patent Application No. 9-116753 maycause a problem of degradation in reproducibility (fading) of alow-intensity character image and an intensity jump in a highlightportion of a photographic image.

In the image forming apparatus disclosed in the above-mentioned JapaneseLaid-Open Patent Application No. 8-340447, since means for determiningan area, in which back projection occurs, is provided, it is consideredthat the accuracy of detection of the area, in which the back projectionoccurs, must be high. In order to achieve such a high-accuracydetection, measures may be taken by performing pre-scanning or usingcharacter and picture separation algorithm. However, it is difficult toapply such a process to a relatively inexpensive apparatus and ahigh-speed machine, which requires real-time processing.

Besides, when performing elimination of back projection, there may be acase in which a low-intensity halftone image is eliminated since thelow-intensity halftone image may be recognized as a back projectionimage.

A description will now be given, with reference to FIGS. 1A, 1B, 2A, 2Band 2C, of such a problem related to a low-intensity halftone image.FIG. 1A shows an example of image data obtained by reading a document ofwhich both sides are printed. When viewing such a document from thefront side, an image printed on the backside of the document may bevisible due to transmission. In such a condition, the intensity of animage (back projection image) of the backside viewed from the front sideis decreased, and an edge of the back projection image may be blunted,and data corresponding to such an image becomes that shown in FIG. 1B.The back projection image can be eliminated by changing datacorresponding to the back projection image to a low intensity valuecorresponding to a background level (white). That is, if a difference inintensity within a small area of the document is below a setting value,the intensity distribution in the small area is regarded as flat, and,at this time, if the intensity of an image in the small area is belowthe setting value, the image is regarded as a back projection image.

However, in a case in which the image on the front side includes alow-intensity image such as a halftone image as shown in FIG. 2A, inparticular, a color image, the halftone image may be recognized as aback projection image and is eliminated from image data to be reproducedas shown in FIG. 2C when an intensity difference of the halftone imageis below a flat detection threshold value B as shown in FIG. 2B. This isbecause the intensity difference (calculated difference) between in asmall area becomes that shown in FIG. 2B, and, therefore, the conditionis established in which the image intensity is below the intensitythreshold value E as shown in FIG. 2B and the intensity difference isbelow the flat detection threshold value B.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an image datacorrecting device, an image reading device and an image formingapparatus in which the above-mentioned problems are eliminated.

A more specific object of the present invention is to eliminate a backprojection image without eliminating an image corresponding to alow-intensity halftone image.

Another object of the present invention is to achieve the elimination ofa back projection image by a single reading operation without usingcomplicated algorithm.

A further object of the present invention is to reproduce a pictureimage without eliminating a low-intensity halftone image.

Still another object is to reduce a background fluctuation afterelimination of a back projection image.

Yet another object is to prevent a crash of dots of a halftone image.

Another object of the present invention is to automatically prevent botha background fluctuation after elimination of a back projection imageand a crash of dots of a halftone image.

In order to achieve the above-mentioned objects, there is providedaccording to one aspect of the present invention an image datacorrecting device comprising: detecting means for detecting an intensitydifference between first image data corresponding to a part of apredetermined small area and second image data corresponding to theremaining parts of the predetermined small area; determining means fordetermining whether the first image data corresponds to a halftoneimage; and intensity changing means for changing an intensity of thefirst image data to a predetermined low intensity, when the intensitydifference is equal to or smaller than a first predetermined value andthe first image data does not correspond to the halftone image and theintensity of the first image data is equal to or smaller than a secondpredetermined value.

According to the above-mentioned invention, the intensity of the imagedata which does not correspond to a halftone image portion can beautomatically changed to the predetermined low intensity when theintensity of the image data is equal to or smaller than the firstpredetermined value and the intensity difference of the image datawithin the predetermined small area is equal to or smaller than thesecond predetermined value since there is a high possibility that suchimage data is generated due to back projection. Since the change ofintensity is not performed when the image data corresponds to a halftoneimage, the image data corresponding to a low intensity halftone image isnot eliminated. Additionally, the elimination of a back projection imagecan be achieved by a single reading operation without using complicatedalgorithm. Further, a picture image can be reproduced withouteliminating a low-intensity halftone image.

In the image data correcting device according to the present invention,the first predetermined value may be determined so that an intensity ofat least a part of an image other than the halftone image is equal to orgreater than the first predetermined value and an intensity of thehalftone image is smaller than the first predetermined value.

Additionally, the second predetermined value may be determined so that adifference between a first intensity difference of the first image datais equal to or greater than the second predetermined value when thefirst image data corresponds to the halftone image, wherein the firstintensity difference is a difference between the intensity of the firstimage data and an average in intensities of the first image data and thesecond image data.

In one embodiment of the present invention, the predetermined small areamay be defined by a pixel matrix, and the first image data correspondsto one of pixels located in the center of the pixel matrix. The pixelmatrix mat be a 3×3 matrix. The predetermined low intensity is equal toor smaller than an intensity of a background of an image from which thepredetermined small area is extracted.

The image data correcting device according to the present invention mayfurther comprise smoothing means for smoothing the first image dataafter the intensity of the first image data is changed. Additionally,the image data correcting device may further comprise selecting meansfor selecting whether to output the first image data before smoothing orafter smoothing. The selecting means may select the first image dataafter smoothing when the first image data corresponds to an image otherthan the halftone image, and selects the first image data beforesmoothing when the first image data corresponding to the halftone image.

Additionally, there is provided according to another aspect of thepresent invention an image reading device comprising: scanning means forscanning an original document to obtain image data and converting theimage data into digital form; and the above-mentioned image datacorrecting device.

Further, there is provided according to another aspect of the presentinvention an image forming apparatus comprises: the above-mentionedimage reading device; the above-mentioned image data correcting device;and an image forming device forming a visible image based on thecorrected image data supplied by the image data correcting device.

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph showing an intensity of images on a front side and abackside of a document; FIG. 1B is a part of the graph of FIG. 1Ashowing an intensity of images on the front side;

FIG. 2A is a graph showing an example of image data including datacorresponding to a halftone image; FIG. 2B is a graph showing anintensity difference calculated based on the image data shown in FIG.1A; FIG. 2C is a graph showing the image data after elimination of aback projection image;

FIG. 3 is an illustration of an interior of an image forming apparatusaccording to a first embodiment of the present invention;

FIG. 4 is a block diagram of an electric system of the copy machineshown in FIG. 3;

FIG. 5 is a block diagram showing an outline of the image processingfunction of IPP;

FIG. 6 is a block diagram showing an outline of the internal compositionof IPP;

FIG. 7 is a block diagram showing an outline of the functionalcomposition of CDIC;

FIG. 8 is a block diagram showing an outline of the functionalcomposition of VDC;

FIG. 9 is a block diagram showing an outline of the functionalcomposition of IMAC;

FIG. 10 is a block diagram showing an outline of the functionalcomposition of FCU;

FIG. 11 shows a flowchart of a process of accumulating image data in MEMand a flowchart of a process of reading the image data from MEM;

FIG. 12 is a block diagram showing a functional composition to perform aback projection correcting process of step Op10;

FIG. 13A is an illustration showing a matrix of pixels used forcomputing an intensity difference; FIG. 13B is an illustration forexplaining the calculation of the intensity difference; FIG. 13C is anillustration for explaining a condition to detect flatness;

FIG. 14A is an illustration for explaining a factor distribution forone-dimensional averaging used by an average computation part shown inFIG. 11; FIG. 14B is an illustration for explaining a factordistribution of a smoothing process performed by a smoothing part shownin FIG. 12;

FIG. 15A is a graph showing an average value of intensity levelsindicated by image data when the image data is that shown in FIG. 2A;FIG. 15B is a graph showing a relationship between a threshold value anda difference value obtained by subtracting an average value from anintensity value when the image data is that shown in FIG. 2A and anaverage value of intensity levels represented by the image data is thatshown in FIG. 15A; FIG. 15C is a graph showing a level of a thresholdvalue and intensity levels of the image data shown in FIG. 2A;

FIG. 16 is a plan view of an operation part of the copy machine shown inFIG. 3;

FIG. 17 is a block diagram of a part of a processor array shown in FIG.6;

FIG. 18 is a block diagram of a back projection correction processingpart, which detects a halftone image based on an edge amount;

FIGS. 19A, 19B, 19C, and 19D are illustrations showing examples of imagedata transmitted to the back projection correction processing part;

FIG. 20 is an illustration for explaining a Laplacian filter used forextracting an edge of an image;

FIG. 21A is a graph showing image data of a front side; FIG. 21B is agraph showing smoothed image data corresponding to an image on a frontside; and

FIG. 22 is an illustration showing a structure of a smoothing filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the drawings, of afirst embodiment of the present invention. FIG. 3 is an illustration ofan interior of an image forming apparatus according to the firstembodiment of the present invention. The image forming apparatusaccording to the present embodiment is a digital full-color copymachine.

In FIG. 3, a color image scanning device (hereinafter, referred to as ascanner) 200, which is an image reading device, scans a document placedon a contact glass 202 by a light emitted by an illumination lamp 105.The light is incident on a color sensor 207 through a group of mirrors204A, 204B and 204C and a lens 206. The incident light is read by thecolor sensor 207 for each color spectrum such as, for example, blue(hereinafter abbreviated as B), green (hereinafter abbreviated as G) andred (hereinafter abbreviated as R), and is converted into electric imagesignals. The color sensor 207 is constituted by a 3-line CCD sensor,which can read B, G, R images on an individual color basis. Thethus-obtained image data is subjected to a color conversion process byan image processing unit (not shown in the figure) based on a colorresolution image signal intensity level of each B, G, R image dataobtained by the scanner 200 so as to obtain color image data, whichincludes recording color information regarding black (hereinafterabbreviated as Bk), cyan (hereinafter abbreviated as C), magenta(hereinafter abbreviated as M) and yellow (hereinafter abbreviated asY).

Using the thus-obtained color image data, images corresponding to Bk, C,M and Y are formed on an intermediate transfer belt in a superimposedstate by a color image recording apparatus (hereinafter referred to as acolor printer) 400, and the images are transferred onto a transferpaper. In the scanner 200, upon receipt of a scanner start signalsynchronized with an operation timing of the color printer 400, anillumination/mirror optical system scans the document in a directionindicated by a leftward arrow so as to obtain image data correspondingto one color for each scan. Then, a latent image formed by the imagedata is developed on an individual color basis by the color printer 400,and the developed image is transferred onto the intermediate transferbelt in a superimposed state so as to form a full-color image consistingof four colors.

The color printer 400 includes a writing optical unit 401 as an exposuremeans. The writing optical unit 401 converts the color image datasupplied by the scanner 200 into optical signals so as to performoptical writing corresponding the original image so as to form latentimages on a photoconductive drum 414. The writing optical unit 401comprises: a laser generating device 441; a light emission drive controlpart (not shown in the figure) which drives the laser generating device441; a polygon mirror 443; a motor 444 for rotating the polygon mirror443; an fθ lens 442; and reflective mirror 446. The photoconductive drum414 rotates in the counterclockwise direction as shown by an arrow inthe figure. A photoconductor cleaning unit 421, a discharge lamp 414M, acharger 419, a potential sensor 414D which detects a latent imagepotential on the photoconductive drum, a selected developer of arevolver type development device 420, a development concentrationpattern detector 414P and an intermediate transfer belt 415 are arrangedaround the photoconductive drum 141.

Revolver development equipment 420 comprises a BK-development machine420K, a C-development machine 420C, an M-development machine 420M, aY-development machine 420Y and a revolver rotation drive part (not shownin the figure) that rotates each development machine in acounterclockwise direction as indicated by an arrow. Each developmentmachine comprises development sleeves 420KS, 420CS, 420MS and 420YS,which rotates with the ear of a development agent being contacted on thesurface of the photoconductive drum 414, and a development paddle thatrotates to scope up and agitate the development agent. In a standbystate, the revolver development equipment 420 is set to a position atwhich a development is performed by the BK development vessel 420. Whena copy operation is started, reading of BK image data by the scanner 200starts at a predetermined timing, and the optical writing and latentimage formation by a laser light is performed based on the image data.Hereafter, the latent image formed the BK image data is referred to as aBK latent image. The same process is performed also on each image dataof C, M, and Y. In order to enable the development starting from theleading edge of the Bk latent image, before the leading edge of thelatent image arrives at the development position of the Bk developmentmachine 420K, the rotation of the development sleeve 420KS is started torotate so as to develop the Bk latent image by a Bk toner. Thereafter,the development operation of the Bk latent image area is continued, andwhen the trailing edge of the latent image passes through the positionof the Bk latent image, the revolver development equipment 420 is drivento rotate promptly from the development position by the Bk developmentmachine 420K to a development position of a development machine of thesubsequent color. This rotating operation is made to complete at leastbefore the leading edge of the subsequent image data reaches.

If the formation cycle of an image is started, the photoconductive drum414 is rotated in a counterclockwise direction as indicated by an arrow.The intermediate transfer belt 415 is rotated in a clockwise directionby the drive motor (not shown in the figure). In connection with therotation of the intermediate transfer belt 415, BK toner imageformation, C toner image formation, M toner image formation, and Y tonerimage formation are performed one by one. Finally, a toner image isformed on the intermediate transfer belt 415 in a superimposed state inthe order of BK, C, M and Y. The formation of the BK image is performedas follows. First, a charger 419 uniformly charges the photoconductivedrum 414 by a corona electric discharge at a minus electric charge ofabout −700 V. Thereafter, a laser diode 441 performs raster exposurebased on the Bk signal. Thus, when a raster image is exposed, theelectric charge proportional to an amount of exposure light disappearsin the exposed portion of the photoconductive drum 414 which is chargeduniformly at the beginning, and an electrostatic latent image is formedas a result. The toner in revolver development apparatus 420 is chargedin negative polarity by agitation by a ferrite career. Moreover, the BKdevelopment sleeve 420KS of the present development apparatus is biasedat a potential which is a combination of a negative direct-currentpotential and an alternating current by a power supply circuit (notshown) to a metal base layer of the photoconductive drum 414.Consequently, the toner does not adhere to the portion in which theelectric charge of the photoconductive drum 414 remains, and a portionwithout an electric charge, i.e., the exposed portion, adsorbs the Bktoner. Thereby, a Bk visible image identical to the latent image isformed. The intermediate transfer belt 415 is engaged with a driveroller 415D, a transfer opposite roller 415T, a cleaning opposite roller415C and a group of idle rollers, and is driven to rotate by a drivemotor (not shown in the figure).

Bk toner image formed on the photoconductive drum 414 is transferredonto the surface of the intermediate transfer belt 415, which is movingat a constant speed while contacting the photoconductive drum 414, by abelt transfer corona discharger (hereinafter referred to as a belttransfer part) 416. Hereafter, transfer of the toner image from thephotoconductive drum 414 to the intermediate transfer belt 415 isreferred to as a belt transfer. In order to prepare for re-use of thephotoconductive drum 414, a slight amount of the non-transferredremaining toner on the photoconductive drum 414 is cleaned by a photoconductor cleaning unit 421.

The collected toner is stored in a waste toner tank (not shown) via arecovery pipe.

In addition, a belt transfer picture of 4 color piles is formed on thesame side of the intermediate transfer belt 415 by sequentiallypositioning the toner images of Bk, C, M and Y, which are sequentiallyformed on the photoconductive drum 414. Then, the belt transfer pictureis transferred onto a transfer paper by a corona discharge transferunit. Meanwhile, on the photoconductive drum 414 side, after theformation process of the BK picture is completed, the formation processof the C picture is performed. That is, reading of the C image data bythe scanner 200 starts at a predetermined timing, and the C latent imageis formed by the laser light writing based on the image data. In thedevelopment position, after the trailing edge of the Bk latent imagepassed and before the leading edge of the C latent image reaches, the Cdevelopment apparatus 420C performs a rotating operation of the revolverdevelopment device so as to develop the C latent image by a C toner.Although the development of the C latent image area is continuedthereafter, the revolver development apparatus 420 is driven to send outthe C development apparatus 420C when the trailing edge of the latentimage passes, similar to the case of above-mentioned Bk developmentapparatus.

Then, the following M development apparatus 420M is located in thedevelopment position.

This operation is also carried out before the leading edge of thefollowing M latent image reaches the development part. It should benoted that, in the formation process of each image of M and Y, anoperation of reading the image data, latent image formation, anddevelopment thereof is the same as that of the process of theabove-mentioned Bk image and C image, and descriptions thereof will beomitted.

The belt cleaning device 415U comprises an entrance seal, a rubberplate, a discharge coil and a contact separation mechanism for theentrance seal and the rubber plate. While carrying out belt transfer ofthe image of the second, third and fourth color after carrying out belttransfer of the Bk picture of the first color, the entrance seal, therubber plate, etc. are separated from the intermediate transfer belt bythe blade contact separation mechanism. A paper transfercorona-discharger (hereinafter, referred to as a paper transfer unit)417 applies AC+DC or DC component to a transfer paper and theintermediate transfer belt 415 by the corona discharge system in orderto transfer the superimposed toner image on the intermediate transferbelt 415 onto the transfer paper.

Transfer papers of various sizes are contained in transfer papercassettes 482 in a feed bank. Transfer papers are fed and conveyed byfeed rollers 483 in a direction of a register roller pair 418R from oneof the cassettes which contains the paper of the designated size.

In addition, a paper tray 412 B-2 is provided for manually feeding anOHP paper, a thick paper, etc. The transfer paper is fed from one of thepaper trays by the time when the picture formation is started, and thetransfer paper stands by in the nip part of register roller pair 418R.Then, when the leading edge of the toner image on the intermediatetransfer belt 415 reaches the paper transfer unit 417, the registerroller pair 418R is driven so that the front end of the transfer paperis aligned with the leading edge of the toner image, thereby achievingpositioning of the transfer paper and the toner image. Thus, transferpaper overlaps with the superimposed color image on the intermediatetransfer belt 415, and passes through the paper transfer unit 417connected to a positive potential. At this time, the transfer paper ischarged by the positive electric charge by a corona discharge current,and a large part of the toner image is transferred onto the transferpaper. Then, when the transfer paper passing a separation dischargerconstituted by a discharge brush (not shown) arranged on the left-handside of the paper transfer unit 417, the transfer paper is dischargedand exfoliated from the intermediate transfer belt 415, and moves to apaper conveyance belt 422. The transfer paper on which the 4 colorsuperimposed toner image has been transferred from the surface of theintermediate transfer belt is conveyed to a fixing unit 423 by the paperconveyance belt 422. In the fixing unit 423, the toner image of thetransfer paper is melted and fixed in the nip part formed by a pressingroller 423B and a fixing roller 423A, which is controlled at apredetermined temperature. Thereafter, the transfer paper is sent out ofa main body by a discharge roller pair 424, and is stacked in a copytray (not shown) in a face-up state.

In addition, the surface of the photo conductor drum 414 after the belttransfer is carried out is cleaned by a photoconductor cleaning unit 421which consists of a brush roller, a rubber blade, etc. Moreover, thephotoconductive drum 414 is uniformly discharged by a discharge lamp414M. Additionally, the surface of the intermediate transfer belt 415after transferring the toner image onto the transfer paper is cleanedagain by being pressed by the blade contact separation mechanism of thecleaning unit 415U. In a repeat copy, the operation of the scanner andthe image formation on the photoconductor proceed to a first color imageforming process of the second sheet at a predetermined timing subsequentto the fourth color image forming process of the first sheet. On theother hand, subsequent to the transfer of the 4-color superimposed imageonto the transfer paper, the Bk toner image of the second sheet istransferred in an area cleaned by a belt-cleaning device on the surfaceof the intermediate transfer belt 415. After that, the same operation asthe first sheet is performed.

The color copy machine shown in FIG. 3 is provided with a compoundfunction. That is, the color copy machine shown in FIG. 3 can print outprint data by the color printer 400 when the print data is suppliedthrough LAN or parallel I/F from a host such as a personal computer.Additionally, the color copy machine can also transmit the image dataread by the scanner 200 to a remote facsimile machine. Moreover, thecolor copy machine can also print out received image data. The copymachine is connected to a public telephone network through a yardswitchboard PBX, and is capable of communicating with a remote facsimilemachine or a management server of a service center through the publictelephone network.

FIG. 4 shows the electric system of the copy machine shown in FIG. 3.The scanner 200, which reads an original document, optically condensesthe light of the lamp irradiation reflected by the original document ona light-receiving element 207 by mirrors and lenses in the reading unit4. The light-receiving element (CCD in the present embodiment) isprovided in a sensor board unit (hereinafter abbreviated as SBU). Theimage signal changed into the electric signal by the CCD is convertedinto a digital signal, i.e., the read image data, by SBU. Then, theconverted image signal is output from SBU to a compression/decompressiondata interface control part (hereinafter, abbreviated as CDIC).

That is, the image data output from SBU is input to CDIC. CDIC controlstransmission of the image data between functional devices and a databus. That is, CDIC controls the data transmission between SBU, aparallel bus Pb and an image-signal processing device (hereinafterabbreviates as IPP) with respect to the image data. Moreover, CDICcontrols the image data transmission between the system controller 6,which manages the control of the whole digital copy machine shown inFIG. 4, and the process controller 1. Furthermore, CDIC performscommunication with respect to other controls. The system controller 6and the process controller 1 communicate mutually through the parallelbus Pb, CDIC and a serial bus Sb. CDIC performs data format conversionfor the data interface of the parallel bus Pb and the serial bus Sb ininside thereof.

The read image data from SBU is transmitted to IPP via CDIC. IPPcorrects signal degradation (signal degradation of a scanner system:distortion of the reading image data based on the scannercharacteristic) in association with the quantization to an opticalsystem and a digital signal, and outputs the corrected image data toCDIC again. CDIC transmits the image data to a copy function controllerMFC, and writes the image data in a memory MEM. Alternatively, the imagedata is returned to the processing system for the printer output by IPP.

That is CDIC has a job for accumulating the read image data in thememory MEM so as to reuse the accumulated data and a job for outputtingthe read image data to a video data control (hereafter, referred to asVDC) without accumulating the read image data in the memory MEM andoutputting an image by the laser printer 400. As an example ofaccumulating in the memory MEM, there is a case where a plurality ofcopies are made from one original document. That is, the reading unit 4is operated only once so as to accumulate the read image data in thememory MEM, and the accumulated data is read for a plurality of times.As an example of not using the memory MEM, there is a case where onesheet of original document is copied only once. That is, since what isnecessary is just to process the read image data for a printer output asit is, it is not necessary to perform the writing in the memory MEM.

When not using the memory MEM, the image data transmitted to CDIC fromIPP is again returned to IPP from CDIC. Image quality processing (15 ofFIG. 5) is performed for changing the intensity data obtained by CCDinto area gradation in IPP. The image data after the image qualityprocessing is transmitted to VDC from IPP. Pulse control is performed bythe VDC for the post-processing with respect to dot arrangement andreproducing the dots so as to form a reproduced image on a transferpaper in the image forming unit 5 of the laser printer 400.

When performing additional processing, for example, rotation of animage, composition of an image, etc. to the image data accumulated inthe memory MEM at the time of reading from the memory MEM, the datatransmitted to CDIC from IPP is sent to an image memory access control(hereinafter abbreviates as IMAC) from CDIC via the parallel bus Pb.Based on the control of the system controller 6, IMAC performs an accesscontrol of the memory module MEM (hereinafter abbreviated as MEM),deployment (character code/character bit conversion) of data for a printof an external personal computer (hereinafter, abbreviated as PC), andcompression/decompression of image data for effective use. The data sentto IMAC is accumulated, after being compressed, to MEM, and theaccumulated data is read if needed. After being decompressed, the dataread from MEM is returned to the original image data, and is sent backfrom IMAC to CDIC via the parallel bus Pb.

After transmitting the image data to IPP from CDIC, the image qualityprocessing by IPP and pulse control by VDC are performed, and a visibleimage (toner image) is formed on the transfer paper in the image formingunit 5.

In the flow of the image data, the parallel bus Pb and the bus controlby CDIC realize the compound function of the digital copy machine. Inthe facsimile transmitting function which is one of the copy functions,the read image data obtained by the scanner 200 is subjected to imageprocessing by IPP, and the reading image data is transmitted to afacsimile control unit (hereinafter, abbreviated as FCU) via CDIC andthe parallel bus Pb. Data conversion to the public line communicationsnetwork (hereinafter abbreviated as PN) is performed by FCU, and theimage data is transmitted to PN as facsimile data. In facsimilereception, line data from PN is converted into image data by FCU, and istransmitted to IPP via the parallel bus Pb and CDIC. In this case, anyspecial image quality processing is not performed, but dot rearrangementand pulse control are performed in VDC, and a visible image is formed ona transfer paper in the image forming unit 5.

When a plurality of jobs, for example, a copy function, a facsimiletransceiver function and a printer output function are executedconcurrently, assignment of a right to use the reading unit 4, the imageforming unit 5 and the parallel bus Pb to the jobs are controlled by thesystem controller 6 and the process controller 1. The process controller1 controls the flow of the image data. A system controller controls thewhole system and manages activation of each resource. The function ofthe compound function digital copy machine is selected through theoperation board OPB, and the contents of processing, such as a copyfunction and a facsimile function, are set up.

FIG. 5 is a block diagram showing an outline of the image processingfunction of IPP. The read image data is transmitted to the scanner imageprocessing part 12 from an input I/F (interface) 11 of IPP through SBUto CDIC. Setting a correction of degradation of the image information byreading as a main purpose, a shading correction, a scanner gammacorrection, an MTF correction, etc. are performed in the scanner imageprocessing part 12. In the scanner image processing part 12, size changeprocessing of enlargement/reduction is also performed in addition to thecorrection processing.

After the completion of the correction processing of read image data,the corrected image data is transmitted to CDIC through an output I/F13. Upon reception of the image data from CDIC through an input 1/F 14,area gradation processing is performed in the image quality processingpart 15. The data after the image quality image processing is output toVDC through output I/F 16. The area gradation processing includesconcentration conversion, Dither processing, error diffusion processing,etc., and main processing thereof is area approximation of gradationinformation.

If the image data which has been subjected to the scanner imageprocessing part 12 is accumulated in the memory MEM, various reproducedimages can be checked by changing the processing performed by the imagequality processing part 15. For example, the atmosphere of a reproducedimage can be changed by changing the concentration of a reproduced imageor changing the number of lines of a dither matrix. It is not necessaryto reread the same image by the scanner 200 for each time to change theprocessing, and different processing can be performed any number oftimes by reading the image data stored in the memory MEM.

FIG. 6 is a block diagram showing an outline of the internal compositionof IPP. IPP has a plurality of I/O boards related with data input andoutput with the exterior, and can arbitrarily set up an input and anoutput, respectively. IPP has a local memory group in inside thereof,and controls a memory area to be used and a route of a data path in amemory control part. Input data and output data are stored in the localmemory group assigned as a buffer memory, and control I/F with theexterior. In a processor array part, various kinds of processing areperformed for the image data stored in a local memory, and a result ofoutput is again stored in the local memory. Parameters for theprocessing procedure of a processor are exchanged between a program RAMand a data RAM. The contents of the program RAM and the data RAM aredownloaded from a process controller through the serial I/F. Otherwise,the process controller reads the contents of the data RAM and monitorsprogress of the processing. When the contents of the processing arechanged or the processing form required by the system is changed, thecontents of the program RAM and the data RAM, which are referred to bythe processor array, are updated.

FIG. 7 is a block diagram showing an outline of the functionalcomposition of CDIC. The image data input-and-output control part 21inputs the read image data from SBU, and outputs data to IPP. The imagedata which has been subjected to a scanner image correction by thescanner image processing part 12 of IPP is supplied to the image datainput control part 22. In order to raise the transmission efficiency ofthe input data in the parallel bus Pb, a data compression is performedin a data compression part 23. The compressed image data is sent througha parallel data I/F 25 to the parallel bus Pb. The image data inputthrough the parallel data I/F 25 from the parallel data bus Pb iscompressed for bus transmission, and is elongated by a data extensionpart 26. The elongated image data is transmitted to IPP by an image dataoutput control part 27. CDIC has a conversion function between paralleldata and serial data. The system controller 6 transmits data to theparallel bus Pb, and the process controller 1 transmits data to theserial bus Sb. Parallel/serial data conversion is performed by a dataconversion part 24 and a serial data I/F 29 for communication of twocontrollers 6 and 1. A serial data I/F 28 is provided for IPP, andperforms serial data transmission with IPP.

FIG. 8 is a block diagram showing an outline of the functionalcomposition of VDC. VDC applies additional processes to the image datainput form IPP in accordance with the characteristic of the imageforming unit 5. VDC performs a pulse control of the image signal forrearrangement processing of the dot by an edge smoothing process and dotformation. Image data is output for the image forming unit 5. Besidesthe conversion of image data, VDC has the format conversion functions33-35 for parallel data and serial data, and VDC alone can also respondto communication of the system controller 6 and the process controller1.

FIG. 9 is a block diagram showing an outline of the functionalcomposition of IMAC. In a parallel data I/F 41, input and output of theimage data to the parallel bus Pb are managed, and storage/reading ofimage data in the memory MEM and development of code data mainly inputfrom an external PC to image data is controlled. The code data inputfrom PC is stored in a line buffer 42. That is, storage of data in alocal area is performed, and the code data stored in the line buffer isdeveloped to image data in a video control part 43 based on based on adeployment processing command input from the system controller 6 via asystem controller I/F 44. The developed image data or the image datainput from the parallel bus Pb through the parallel data I/F 41 isstored in the memory MEM. In this case, the image data, which serves asa candidate to be stored, is selected by a data conversion part 45.Then, a data compression is performed in a data compression part 46 soas to raise an efficiency of memory use, and the image data is stored inthe memory MEM while managing addresses of the memory MEM by a memoryaccess control part 47. Reading of the image data stored in MEM isperformed based on a readout address controlled by the memory accesscontrol part 47. The read image data is decompressed by a datadecompression part 48. When transmitting the decompressed image data tothe parallel bus Pb, the data transmission is performed through theparallel data I/F 41.

FIG. 10 is a block diagram showing an outline of the functionalcomposition of FCU. The facsimile transceiver part FCU changes imagedata into communication form, and transmits to the external line PN.Moreover, FCU restores data from the external circuit PN to image data,and carries out a record output by the image forming unit 5 through anexternal I/F 51 and the parallel bus Pb. The facsimile transceiver partFCU comprises a facsimile image processing part 52, an image memory 53,a memory control part 55, a facsimile control part 54, an imagecompression/decompression part 56, a modem 57 and a network control unit58. Among these parts, a binary value smoothing processing is performedin the edge smoothing processing 31 of VDC with respect to the facsimileimage processing performed by the facsimile image processing part 52.Moreover, with respect to the image memory 53, a part of an outputbuffer function is compensated by IMAC and MEM.

In the thus-constituted facsimile transceiver part FCU, when startingtransmission of image information, the facsimile control part 54 sendsan instruction to the memory control part 55 so as to sequentially readthe picture information accumulated in the picture memory 53. The readpicture information is restored to the original signal, and densityconversion processing and size change processing are made, and the readpicture information is supplied to the facsimile control part 54. Theimage signal supplied to the facsimile control part 54 is encoded andcompressed by the image compression/decompression part 56. Thecompressed image signal is sent to a destination address through thenetwork control unit 58, after becoming modulated by the modem 57. Theimage information of which transmission is completed is deleted from thepicture memory 53.

At the time of reception, a received image is once accumulated in theimage memory 53. If a record output of the received image is possible,the received image is output when an image corresponding to one sheet iscompleted. Moreover, when a call is received during a copy operation andreception of image data is started, the received image data isaccumulated in the image memory 53 until the rate of use of the imagememory 53 reaches a predetermined value, for example, 80%. When the rateof use of the image memory 53 reaches 80%, a write-in operationcurrently performed is interrupted compulsorily, and the received imagedata is read from the image memory 53 so as to output the image data byrecording. The received image data read from the image memory 53 at thistime is deleted from the image memory 53. Then, the interrupted write-inoperation is resumed when the rate of use of the picture memory 53 fellto a predetermined value, for example, 10%. Thereafter, when the wholeof the write-in operation is ended, the record output of the remainingreceived image is carried out. Additionally, after interrupting thewrite-in operation, various parameters for the write-in operation at thetime of interruption are internally evacuated so that the write-inoperation can be resumed by internally returning the parameters at thetime of resumption.

In the above-mentioned example, CDIC which is a picture bus managementmeans and IMAC which is a memory management means are connected by theparallel bus Pb which includes one set of image buses. SBU as imagereading means, VDC as write-in means and IPP as image signal processingmeans are not connected to the image bus Pb directly but connected tothe image bus management means CDIC. For this reason, the use managementof the image bus Pb is performed only by the image bus management meansCDIC and the memory management means IMAC. Therefore, an arbitration anda transmission control of the bus Pb are easy and efficient.

FIG. 11A is a flowchart of a process of accumulating image data in MEM,and FIG. 11B is a flowchart of a process of reading the image data fromMEM. FIG. 11A shows the process (steps Ip1-Ip13) applied to the imagedata until the image data, which is generated by the image scanner 200,is written in MEM. FIG. 11B shows the process (steps Op1-Op13) appliedto the image from reading the image data from MEM and until the imagedata is output to the printer 400. The data flow between such a bus anda unit is controlled by a control of CDIC. With respect to the readimage data, the scanner image processing Ip1-Ip13 (12 of FIG. 5) isindependently performed, and with respect to the image data to be outputto the printer 400, the image quality processing Op1-Op13 (15 of FIG. 5)by IPP is independently performed.

In the present embodiment, the “reverse side projection correctingprocess” of step Op10 in FIG. 11B is performed in the process of stepsOp1-Op13 in which the image data is read from MEM and is output to theprinter 400. FIG. 12 is a block diagram showing a functional compositionto perform the reverse side projection correcting process of step Op10.The image data of each pixel is written in an intensity differencedetecting matrix 61 of a flat detection 60, which is a 3×3-pixel matrixhaving an attention pixel (intensity value e) in the center to which thepixel data Din currently being supplied is provided. If the intensityvalues represented by the image data are made into a-i as shown in FIG.13A, the flat detection 60 computes the intensity differences A (A1-A8)within the pixel matrix 61 as shown in FIG. 13B. Then, a comparator 62checks whether each of the computed intensity values is smaller than aflat detection threshold value B. If the intensity differences (all ofA1-A8) are smaller than the threshold value B, a flat detection signal Cis set to a high level 1 which represents flatness. If any one of theintensity differences is equal to or greater than the threshold value B,the flat detection signal C is cleared to a low level 0 which representsnon-flatness. The flat detection signal C is one of the inputs of alogical product operator 69.

When the image data includes data representing dots of a halftone imageas shown in FIG. 2A, the intensity difference A and the threshold valueB have a relative relationship as shown in FIG. 2B.

In an intensity detection part 63, a comparator 64 checks whether or notan intensity value e=D of an attention pixel is smaller than thethreshold value E for low-intensity detection. When the intensity valuee=D is smaller than the threshold value E, a low intensity detectionsignal F is set to a high level 1, which represents that the intensityof the attention pixel is low. When the intensity value e=D is equal toor greater than the threshold value E, a low intensity detection signalF is cleared to a low level 0, which represents that the intensity ofthe attention pixel is not low. The low intensity detection signal F isone of the inputs of the logical product operator 69. FIG. 15C is agraph showing the level of the threshold value E and intensity levels ofthe image data shown in FIG. 2A.

In a halftone detection part 65, an average computing part 66 computesthe average value of the image data which consists of 8 pixelscontaining the attention pixel and seven pixels adjacent to theattention pixel. Namely, as shown in FIG. 14A, each pixel data of theeight pixels is multiplied by the same factor 1, and the product isdivided by 8 (remove the lower three bits). This process is equivalentto a one-dimensional smoothing filter process. FIG. 15A is a graphshowing the average value of intensity levels indicated by image datawhen the image data is that shown in FIG. 2A.

In the halftone detection part 65, a difference value G, which isobtained by subtracting the average value from the intensity valueindicated by the image data of the attention pixel, is compared with athreshold value H for halftone detection by a comparator 68. When thedifference value G is smaller than the threshold value H, a non-halftonedetection signal I is set to a high level 1, which represents that theattention pixel does not correspond to a dot of a halftone image. Whenthe difference value G is equal to or greater than the threshold valueH, the non-halftone detection signal I is cleared to a low level 0,which represents that the attention pixel corresponds to a dot of ahalftone image. The non-halftone detection signal I is also one of theinputs of the logical product operator 69.

FIG. 15B is a graph showing a relationship between the threshold valueand the difference value G obtained by subtracting the average valuefrom the intensity value when the image data is that shown in FIG. 2Aand the average value of the intensity levels represented by the imagedata is that shown in FIG. 15A. An output J of the logical productoperator 69 becomes the high level 1, when three signals, that are theflat detection signal C, the low intensity detection signal F and thenon-halftone detection signal I are at the high level 1. That is, whenan image intensity distribution is flat, and the image intensity of anattention pixel is low and does not correspond to a dot of a halftoneimage, it can be regarded as back projection. The output J of thelogical product operator 69 at this time becomes the high level 1, whichrepresent that the attention pixel corresponds to back projection.

The image data of an attention pixel is given to an intensity changepart 70. When the output J of the logical product operator 69 is at thehigh level which represents the back projection, the intensity changepart 70 changes the image data of the attention pixel to a predeterminedlow level K corresponding to a background level, and the image data isoutput from the intensity change part 70 as an output L. When the outputJ of the logical product operator 69 is at the low level 0 (non-backprojection), the intensity change part 70 outputs the image data of theattention pixel as the output L without applying the change.

When the image data is that shown in FIG. 2A, and if a logical productof the detection output C of the flat detection part 60 and thedetection output F of the intensity detection part 63 is given to theintensity change part so as to select one of the original image data Dinand the background level setting value K in accordance with the level ofthe output L of the intensity change part 70, the output L of theintensity change part 70 may eliminate a low intensity dot as shown inFIG. 2C. However, in the present embodiment, since the non-halftonedetection signal I of the halftone detection part 65 is set as one ofthe inputs of the logical product operator 69, when the image data isthat shown in FIG. 2A, the output L of the intensity change part 70becomes what leaves a low intensity dot as it is as shown in FIG. 15C,thereby achieving a high reliability of the back projection correctingprocess.

The image data L output by the intensity change part 70 outputs is givento a smoothing part 71 and an output selector 72. The smoothing part 71applies a smoothing filter process to the image data L using a filtercoefficient shown in FIG. 14B. According to the pixel matrixdistribution of the filter coefficient shown in FIG. 14B, a high weightis given to an attention pixel. Moreover, a next high weight is given tothe maximum proximity pixels having a side contacting the attentionpixel. Furthermore, a low weight is given to the proximity pixels havinga corner contacting the attention pixel. Therefore, the smoothing part71 computes the weighted average value of the image data of a small areahaving the attention pixel at the center, and, therefore, the intensitylevel of the circumference pixel is reflected in the intensity level Mof the attention pixel.

The output I of the halftone detection part 65 is also given to thesmoothing selection part 73. An automatic/selection signal, whichrepresents whether an automatic or a selection is designated through theoperation part OPB, and a character/halftone signal, which representwhether a character or a halftone is designated, are also given to thesmoothing selection part 73. A high level 1 of the 1-bitautomatic/selection signal indicates that the “automatic” is designated,and a low level 0 indicates that the “selection” is designated. A highlevel 1 of the 1-bit character/halftone signal indicates that the“character” is designated, and a low level 0 indicates that the“halftone” is designated. A description will be given later, withreference to FIG. 16, of the setting of the automatic/selection signaland the character/halftone signal.

When the automatic/selection signal is at the high level 1 whichdesignates the “automatic”, an AND gate 74 is turned on, an AND gate 75is turned off and the non-halftone detection signal I of thenon-halftone detection part 65 is given to the selector 72 via an ORgate 76. The selector 72 selects the smoothed image data M as an outputof the back projection correcting process (step Op10) when thenon-halftone detection signal I is at the high level 1, which representsthat the attention pixel does not correspond to a dot of a halftoneimage. On the other hand, the selector 72 selects the non-smoothed imagedata L=N as an output of the back projection correcting process (stepOp10) when the non-halftone detection signal I is at the low level 0,which represents that the attention pixel corresponds to a dot of ahalftone image.

When the automatic/selection signal is at the low level 0 whichdesignates the “election”, the AND gate 74 is turned off, the AND gate75 is turned on and character/halftone signal is given to the selector72 via the AND gate 75 and the OR gate 76. The selector 72 selects thesmoothed image data M as an output of the back projection correctingprocess (step Op10) when the character/halftone signal is at the highlevel 1, which represents that the attention pixel correspond to acharacter image. On the other hand, the selector 72 selects thenon-smoothed image data L=N as an output of the back projectioncorrecting process (step Op10) when the character/halftone signal is atthe low level 0, which represents that the attention pixel correspondsto a dot of a halftone image.

According to the above-mentioned process, an intensity change isperformed based on the result of detection of the flat detection part 60and the intensity detection part 63 when a halftone image is notdetected by the halftone detection part 65, and, thereby, backprojection is eliminated.

By applying the smoothing process by the smoothing part 71 to the outputL of the intensity change part 70, an intensity discontinuous part afterthe intensity change is equalized, thereby improving the image quality.However, if the smoothing process is applied to data corresponding tothe low intensity halftone portion, the data is blunted and an intensityfall is caused. In the worst case, the data may be eliminated.Therefore, in the present embodiment, whether to render the output O tobe the smoothed data M or non-smoothed data L=N is changed by theselector 72. The smoothing selection part 73 selects the non-smootheddata for a halftone image so as to avoid an unnecessary intensity fall.When “automatic” is designated, the smoothing selection part 73automatically switches the selector according to the detection result Iof the halftone detection part 65. Therefore, the present embodiment cancope with a case where a low intensity halftone image and other imagesare present in the same original image. If an operator does not wish toapply the smoothing process, the operator may designate both “selection”and “halftone”. In such as case, the smoothing selection part 73 setsthe selector 72 to select the image data L=N as the output 0. If theoperator wishes to apply the smoothing process, the operator maydesignate both “selection” and “character”. In such as case, thesmoothing selection part 73 sets the selector 72 to select the smoothedimage data M as the output O.

As shown in FIG. 16, the operation part OPB is provided with a liquidcrystal touch panel 79, a ten-key pad 80 a, a clear/stop key 60 b, astart key 60 c, a mode clear key 60 e, a test print key 80 f and afunction selection key 80 g for selecting a copy function, a scannerfunction, a print function, a facsimile function, an accumulationfunction, an edit function, a registration function and other functionsand indicating a state of execution of these functions. An input/outputscreen, which corresponds to a function designated through the functionselection key 80 g, is displayed on the liquid crystal touch panel 79.For example, when the copy function is designated, a message, whichshows the state of function key 79 a, a number of copies and a state ofthe image forming apparatus, is displayed. The test print key 80 f is akey for printing only one set of print irrespective of the number ofsets of print, which has been set, so as to check the result of print.

A character designation key 80 h is operated when an operator regards anoriginal image as a binary image such as a text or a line drawing. Ahalftone designation key 80 i is operated when an operator regards anoriginal image as a halftone image or a gray scale image such as aphotograph or a picture. When the “character” is designated, data (1-bitsignal), which represents the designation, is set to a high level 1.When the “halftone” is designated, the data (1-bit signal), whichrepresents the designation, is set to a low level 0. When the“character” is designated, IPP applies a process to the image data so asto clearly display characters and lines. When the “halftone” isdesignated, IPP applies a process to the image data so as to smoothlydisplay a gradation image such as a photograph.

Moreover, when an initial-setting key 80 d is operated, a selection menuis displayed for selecting an “initial value setting” function forsetting up various initial values, an “ID setting” function, a“copyright registration/setting” function and an “output of operationhistory” function. The “initial value setup” includes a setup of the“automatic/selection.” The “automatic/selection” is set whether toselect automatically whether or not the smoothing process of thesmoothing part 71 be applied to the image data, which has passed throughthe intensity change part for the back projection correction ordesignation by the character designation key 80 h by an operator or bythe halftone designation key 80 i, that is whether to select accordingto a selection made by the operator. The data (1-bit signal)representing the setting is at a high level 1 when the “automatic” isset, and is at a low level 0 when the “selection” is set.

FIG. 17 is a block diagram showing an outline composition of a SIMD typeprocessor for image processing adopted in IPP. A SIMD type processor isa processor, which executes a single command with respect to a pluralityof sets of data. In the present embodiment, the processor comprises aplurality of processor elements PE1-PE8 (8 processors for 1 byteparallel processing). Each of the processor elements PE1-PE8 comprises aregister (Reg) for storing data, a multiplexer (MUX) for accessing theregister (Reg) of other PE, a barrel shifter (ShiftExpand), a logicoperation unit (ALU), an accumulator (A) which stores a result ofoperation and a temporary register (F) that temporarily evacuates thecontents of the accumulator (A). Each register is connected to anaddress bus and a data bus, and stores a command code which specifiesprocessing or data to be processed.

The data set as an object to be processed by the register is input tothe logic operation unit ALU, and the result of operation is stored inthe accumulator A. In order to take out the result of the processorelement PE, the result of processing is temporarily evacuated to thetemporary register F. Thus, the result of processing of the object datais obtained by taking out the contents of the temporary register F. Acommand code is given to each of the processor elements PE1-PE8 with thesame contents. The object data of processing is given in a differentstate for each of the processor elements PE1-PE8. By referring to thecontents of Reg of the adjacent PE by the multiplexer MUX, the operationresult is processed in parallel and is output to each accumulator A. Forexample, if the contents of the image data of one line are arranged toPE for each pixel and operation processing is carried out with the samecommand code, the processing result for 1 byte can be obtained for ashorter time than a case in which serial processing is carried out on anindividual pixel basis. As mentioned above, the image data processing iscarried out by the processor elements PE1-PE8 in IPP.

A description will now be given, of another example of the halftonedetection process applicable to the above-mentioned back projectioncorrecting process. In this example, detection is made as to whether animage corresponds to a halftone image or other images based on a changein intensity of an edge of the image. A degree of change in intensity ofan edge of an image is hereinafter referred to as an edge amount.

As mentioned above, the back projection correcting process is performedby the image quality processing part 15 of IPP shown in FIG. 5. A backprojection correction processing part 150 shown in FIG. 18 is providedin the image quality processing part 15. FIG. 18 is a block diagram ofthe back projection correction processing part 150 which detects ahalftone image based on the edge amount. FIGS. 19A, 19B, 19C, and 19Dshow an example of the image data transmitted to the back projectioncorrection processing part 150. FIG. 19A shows a front-side image 81extracted from image data of a front side of an original document. FIGS.19B, 19C and 19D show reverse-side images 82 a, 82 b, and 82 c extractedfrom image data of a reverse side of the original document.

The front-side image 81 shown in FIG. 19A includes a transmissioncomponent (a back projection image) other than the image printed on thefront side of the original document. The transmission component is theimages printed on the reverse side and appears on the front side due totransmission. The image 82 a shown in FIG. 19B corresponds to a halftoneimage, the image 82 b shown in FIG. 19C corresponds to a characterimage, and the image 82 c shown in FIG. 19D corresponds to a solid orthick image. In FIGS. 19A through 19D, the horizontal axis represents aposition and the vertical axis represents an intensity value of theimage data.

As shown in FIG. 18, the back projection correction processing part 150comprises an edge amount detection part 151, a smoothing part 152, adetermining part 153 and an intensity correction part 154. The edgeamount detection part 151 extracts an edge portion of the transmittedimage data. The smoothing part 152 smoothes the result of edgeextraction. The determining part 153 distinguishes a back projectionimage from images printed on the front side of the original document.The intensity correction part 154 performs the elimination of image datacorresponding to the back projection image based on the result ofdetermination of the determining part 153.

When image data is transmitted to the back projection correctionprocessing part 150, the edge amount detection part 151 detects the edgeamount of the image data. For example, even if it is an acute image onthe reverse side, it becomes an image having a dull edge due totransmission to the front side. On the other hand, a printed imagehaving a low contrast, for example, a character portion or a halftoneportion has a sharp edge even if an average intensity is low.Accordingly, the image data generated by back projection can bedistinguished from the image data generated by the true image on thefront side based on the existence of a sharp edge. When detecting theexistence of the edge, the image data of the front-side picture 81 shownin FIG. 19A is processed by a Laplacian filter L1 shown in FIG. 20 so asto extract the edge portion 83 of the image on the front side as shownin FIG. 21A. Thereby, the edge portion of a back projection image can bedistinguished from the edge portion of a low contrast image.

Thus, when the edge portion 83 is extracted, the inner side of acharacter may be judged to be a back projection image since the innerside of a character is not judged to be an edge. Then, the smoothingpart 152 obtains a smoothed image 84 of the front-side image 81 as shownin FIG. 21B by smoothing the result of edge extraction by the amountdetection part 151 by a smoothing filter L2 shown in FIG. 22. Thedetermining part 153 detects a character portion of a low contrast imageportion and a halftone portion by binarizing the smoothed image data.Therefore, the halftone detection process according to theabove-mentioned edge amount detection is applicable to the halftonedetection process performed by the halftone detection part 65 shown inFIG. 12.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority application No.2000-266591 filed on Sep. 4, 2000, the entire contents of which arehereby incorporated by reference.

1. An image data correcting device for correcting a reverse-sideprojection image that is generated when reading a document from one sideof the document, the document having images on both sides thereof, theimage data correcting device comprising: a flat detection partconfigured to detect an intensity difference between first image datacorresponding to a part of a predetermined small area and second imagedata corresponding to the remaining parts of the predetermined smallarea, and compare the intensity difference with a first predeterminedvalue; an average computing part configured to compare a subtractionvalue with a second predetermined value, the subtraction value beingobtained by subtracting an average value of the first image data and thesecond image data from the intensity of the first image data; anintensity detection part configured to compare the intensity of thefirst image data with a third predetermined value; and an intensitychanging part configured to change an intensity of the first image datato a predetermined low intensity, when the intensity difference is equalto or less than a first predetermined value and the subtraction value isequal to or less than the second predetermined value and the intensityof the first image data is equal to or less than the third predeterminedvalue, thereby correcting the reverse-side projection image.
 2. Theimage data correcting device as claimed in claim 1, wherein the secondpredetermined value is determined so that an intensity of at least apart of an image other than the halftone image is equal to or greaterthan the second predetermined value and an intensity of the halftoneimage is less than the second predetermined value.
 3. The image datacorrecting device as claimed in claim 1, wherein the first predeterminedvalue is determined so that a first intensity difference of the firstimage data is equal to or greater than the first predetermined valuewhen the first image data corresponds to the halftone image, wherein thefirst intensity difference is a difference between the intensity of thefirst image data and an average in intensities of the first image dataand the second image data.
 4. The image data correcting device asclaimed in claim 1, wherein the predetermined small area is defined by apixel matrix, and the first image data corresponds to one of pixelslocated in the center of the pixel matrix.
 5. The image data correctingdevice as claimed in claim 4, wherein the pixel matrix is a 3×3 matrix.6. The image data correcting device as claimed in claim 1, wherein thepredetermined low intensity is equal to or less than an intensity of abackground of an image from which the predetermined small area isextracted.
 7. The image data correcting device as claimed in claim 1,further comprising smoothing means for smoothing the first image dataafter the intensity of the first image data is changed.
 8. The imagedata correcting device as claimed in claim 7, further comprisingselecting means for selecting whether to output the first image databefore smoothing or after smoothing.
 9. The image data correcting deviceas claimed in claim 8, wherein the selecting means selects the firstimage data after smoothing when the first image data corresponds to animage other than the halftone image, and selects the first image databefore smoothing when the first image data corresponds to the halftoneimage.
 10. An image reading device a document having images on bothsides thereof, the image data reading device comprising: a scanning partconfigured to scan an original document to obtain image data andconverting the image data into digital form; and an image datacorrecting device configured to correct a reverse-side projection imagethat is generated when reading the document from one side of thedocument the image data supplied by the scanning means, wherein theimage data correcting device comprises: a flat detection part configuredto detect an intensity difference between first image data correspondingto a part of a predetermined small area and second image datacorresponding to the remaining parts of the predetermined small area,and compare the intensity difference with a first predetermined value;an average computing part configured to compare a subtraction value witha second predetermined value, the subtraction value being obtained bysubtracting an average value of the first image data and the secondimage data from the intensity of the first image data; an intensitydetection part configured to compare the intensity of the first imagedata with a third predetermined value; and an intensity changing partconfigured to change an intensity of the first image data to apredetermined low intensity, when the intensity difference is equal toor less than a first predetermined value and the subtraction value isequal to or less than the second predetermined value and the intensityof the first image data is equal to or less than the third predeterminedvalue, thereby correcting the reverse-side projection image.
 11. Animage forming apparatus comprising: an image reading device generatingimage data by scanning an original document, the document having imageson both sides thereof, said image reading device including an image datacorrecting device configured to correct a reverse-side projection imagethat is generated when reading the document from one side of thedocument the image data supplied by the image reading device; and animage forming device forming a visible image based on the correctedimage data supplied by the image data correcting device, wherein theimage data correcting device comprises: a flat detection part configuredto detect an intensity difference between first image data correspondingto a part of a predetermined small area and second image datacorresponding to the remaining parts of the predetermined small area,and compare the intensity difference with a first predetermined value;an average computing part configured to compare a subtraction value witha second predetermined value, the subtraction value being obtained bysubtracting an average value of the first image data and the secondimage data from the intensity of the first image data; an intensitydetection part configured to compare the intensity of the first imagedata with a third predetermined value; and an intensity changing partconfigured to change an intensity of the first image data to apredetermined low intensity, when the intensity difference is equal toor less than a first predetermined value and the subtraction value isequal to or less than the second predetermined value and the intensityof the first image data is equal to or less than the third predeterminedvalue, thereby correcting the reverse-side projection image.